Mdependent

I have added one new covariance structure that may be called by the user using MDependent(m) as their covstruct argument for the Generalized Estimating Equations class. Here is a brief explanation of this structure:

1) It assumes the correlation between all observations within m time steps from each other and in the same cluster is a constant r.
2) It assumes all other correlations between observations is 0.
3) The number m may range from 0 to positive infinity.

Locally tested for replicability, equivalence for m = inf to Exchangeable case, and m = 0 to Independence case.